A known type of reflective liquid crystal display comprises a liquid crystal layer disposed between a linear polarizer and a reflector. The liquid crystal layer is switchable between a first state, in which polarized light from the polarizer is incident on the reflector with linear polarization, and a second state, in which light from the polarizer is incident with circular polarization on the reflector. When linearly polarized light is incident on the reflector, it is reflected without effectively changing its polarization azimuth. The display or picture element (pixel) of the display in this mode is reflective and appears bright. In the second mode, the circularly polarized light is reflected by the reflector with the opposite sense of circular polarization. In this case, the display or the pixel appears dark. The liquid crystal layer may provide linear polarization in the absence of an applied electric field across the layer, in which case the display is of the normally white type. Alternatively, the liquid crystal layer may provide circular polarization in the absence of an applied field, in which case the display is of the normally black type.
A known display of this type has a twisted nematic liquid crystal layer. Beynon et al, Proceeding of the International Display Research Conference, 1997, L-34 relates to this type of display and derives values for the liquid crystal parameters of retardation, twist and alignment orientation necessary to produce circular polarization by the layer. Most of these sets of parameters are such that the display brightness approaches a maximum value asymptotically as the electric field across the liquid crystal layer approaches infinity. Thus, the maximum brightness white state cannot be achieved for a finite applied field.
U.S. Pat. No. 5,490,003 and Yang, Euro Display 1996, pp 449 disclose an arrangement where the maximum brightness can be achieved. However, this arrangement has a somewhat chromatic dark state and the maximally bright state occurs at a relatively high voltage.
Untwisted liquid crystal displays of this type exhibit the same problem of being unable to achieve the maximum brightness white state for finite applied field. U.S. Pat. No. 4,767,190 discloses an arrangement of this type in which an additional fixed retarder of relatively low retardation is disposed optically adjacent the untwisted liquid crystal layer with the optic axes of the retarder and the layer crossed. The retardation of the additional retarder is therefore effectively subtracted from the retardation provided by the liquid crystal layer and this allows the maximum brightness white state to be achieved for a finite applied field. However, this technique is not applicable to twisted liquid crystal displays because the direction of the director, and hence the optic axis, varies azimuthally so that perfect cancellation of retardation cannot be achieved.
In order to achieve a high contrast ratio (the ratio of light reflected by the display in its maximal reflection state to light reflected by the display in its darkest state), the dark state should be made as achromatic as possible. This requires that the light incident on the reflector for the dark state should be as close to perfectly circularly polarized as possible for as much of the visible spectrum as possible. In order to improve the achromaticity of the dark state, it is known to provide extra retarders which are optically in series with the twisted liquid crystal layer, generally between the layer and the linear polarizer. For instance, it is known to provide an achromatic quarter wave retarder between the polarizer and the twisted liquid crystal layer as disclosed in Wu et al, Appl. Phys. Lett., 1996, 68, pp 1455.
In the display or pixel dark state, the liquid crystal layer should have minimal effect on the polarization of light from the retarder so that the achromaticity of the display should be determined by the achromatic quarter wave retarder. In the bright state, the liquid crystal layer converts the circular polarization produced by the quarter wave retarder to linear polarization. However, the liquid crystal layer only has zero retardation at infinite voltage so that there is some residual retardation which degrades the achromaticity of the dark state and requires as large a voltage as possible to achieve the best possible dark state. Further, most achromatic quarter wave retarders are formed from combinations of retarders and are therefore usually circular polarizers rather than quarter waveplates, which compounds the problem further.
In the case of twisted liquid crystal layers, the only exception to the problem of residual retardation is in the case where the twist of the liquid crystal is 900. In this case, the surface portions of the liquid crystal layer approximately cancel each other when the voltage and hence applied field across the layer exceeds a predetermined value (assuming equal pretilts at both liquid crystal layer surfaces). For instance, Wu et al, Appl. Phys. Lett., 1996, 68, pp 1455, discloses a twisted nematic liquid crystal display of this type including an achromatic retarder. However, such a display cannot achieve as bright a white state as displays in which the liquid crystal twist is different from 90° except for much larger retardation.
The term “optic axis” as used herein refers exclusively to the slow axis of a birefringent material.